A “plasma based semiconductor process” generally refers to a methodology for fabricating microelectronic devices such as very large scale integration (VLSI) microelectronic chips and/or thin film transistors (TFTs). In particular, plasma based semiconductor processes may be used, for example, in deposition processes (e.g., plasma enhanced chemical vapor deposition, hereinafter “PECVD”) and/or etching processes during the fabrication of microelectronic devices.
For example, a PECVD run (a particular instance of conducting a PECVD process) includes the following steps: 1) a device to be processed is placed within a chamber; 2) an initial condition is created inside the chamber using control parameters (e.g., RF power, electrode spacing, gas pressure, SiH4 flow, N2O flow, etc.); 3) plasma is ignited; 4) the control parameters are adjusted; 5) a desired end point (e.g., a predetermined thickness of a film being deposited) is reached; and 6) the run is then terminated.
During a PECVD run, diagnosis and proper process control of the PECVD run are desired in order to ensure that microelectronic devices produced by the PECVD run are free of defects. The diagnostics and proper process control may be provided manually or automatically in order to determine when an end point has been reached in order to adjust the control parameters while the PECVD run is in progress.
A first group of conventional methods for controlling a PECVD run focus on determining when its end point is reached. There are three general techniques in this group of conventional methods: (1) optical end point; (2) interferometric end point; and (3) test wafer measurement.
The optical end point technique involves determining the end point of a PECVD run by monitoring one or two narrow spectral bands of spectral emission from the plasma of the PECVD run. This technique is generally not predictive. In particular, use of this technique does not adequately permit detection of an approaching end point. Thus, a PECVD run must first come to its end point before the end point is observed by this technique, which almost always causes delays in terminating the PECVD run (which, e.g., can then result in the endpoint being overshot). While these shortcomings may be overlooked in experimental PECVD runs, they may cause unacceptable level of errors should this technique be used in manufacturing processes.
The interferometric end point technique also attempts to determine the end point, but uses interferometric interference fringes as a measurement. However, a number of different types of material (e.g., metal) do not show the interferometric interference fringes unless the film deposited by the PECVD run is extremely thin. Hence, this technique may not be viable for depositing metal films. In addition, similar to the optical end point technique described above, the interferometric technique does not predict the end points, thereby causing delays in stopping a PECVD run when its end point is reached.
The test wafer measurement technique involves determination of the quality of microelectronic devices by physically examining one or more microelectronic devices per a batch of manufactured microelectronic devices. Each time a test microelectronic device is examined and passes a minimum standard (e.g., is determined to be within a predetermined range of the end point thickness), it acts as a certification that microelectronic devices of the batch may also meet the minimum standard. However, when a test microelectronic device fails to meet the minimum standard, each and every one of the microelectronic devices of that batch must be discarded or individually tested, which is an expensive process because each microelectronic device may be worth many tens of thousands of dollars. Another drawback of this technique is the fact that many of the quality tests are destructive in nature.
In addition to the deficiencies mentioned above, the above described conventional techniques of process control cannot detect a PECVD run that has gone out of optimal process specifications (e.g., overshot the optimal thickness of the deposition film) while the process is ongoing. In addition, these techniques do not provide steps that are necessary to correct erroneous processes.
In order to reduce some of the shortcomings of the above-described techniques, a second group of conventional techniques have also been developed. This group of conventional techniques does not focus on determining when the end points have been reached as in the first conventional techniques. Instead, the second group monitors ongoing processes. An example of such techniques involves monitoring the control parameters. This technique has shown some success in predicting film properties for PECVD runs, but it is still relatively inaccurate in determining the end points because this technique relies only on the control parameters without monitoring the actual PECVD run (e.g., without monitoring the progress of the device being produced).
Thus, what is needed is a scheme to better control semiconductor processes so that, e.g., the quality of fabricated microelectronic devices will increase.